Current confinement in semiconductor light emitting devices

ABSTRACT

Semiconductor light emitting devices, lasers and LEDs, are described in which the current flow channel is narrower near the top surface of the device and wider at its bottom near the active region. Also, described are several attenuation masks for fabricating the channels of these devices by particle bombardment.

CROSS REFERENCE TO RELATED APPLICATIONS

This application was concurrently filed with application Ser. No.247,627, U.S. Pat. No. 4,408,331, by R. L. Hartman et al entitled"V-Groove Semiconductor Light Emitting Devices."

BACKGROUND OF THE INVENTION

This invention relates to light emitting semiconductor devices, such aslasers and LEDs, and more particularly to the confinement of currentflow in these devices.

Nearly two decades ago light emitting semiconductor devices, especiallythose having a planar p-n junction in a monocrystalline semiconductorbody, utilized broad area electrical contacts on opposite major surfacesof the body to apply forward bias voltage and pumping current to thejunction. In an LED the resulting radiative recombination of holes andelectrons in the active region in the vicinity of the junction generatedspontaneous radiation. Primarily, one fundamental modification convertedthe LED to a laser: a cavity resonator was formed on the semiconductorbody by a pair of parallel cleaved crystal facets orthogonal to thejunction. When the pumping current exceeded the lasing threshold, thespontaneous radiation, which in the LED was emitted from the activeregion essentially isotropically, was converted to stimulated radiation,which in the laser was emitted as a collimated beam parallel to thejunction and along the resonator axis. Of course, other designconsiderations played a role in making the advance from LED to laser,but these matters are not discussed here inasmuch as our purpose at thispoint is merely to state the now well-known kinship between p-n junctionlasers and LEDs.

The broad area contacts (e.g., 100 μm wide) on these devices caused thepumping current density at the p-n junction to be relatively low which,therefore, meant that relatively high currents (e.g., hundreds of mA inlasers) were required to achieve desirable radiation power levels. Highcurrents in turn heated the semiconductor body and necessitated couplingthe device to a suitable heatsink and/or operation of the device atcryogenic temperatures. The basic solution to this problem was then, andis today, to reduce the area of the p-n junction which has to be pumpedso that for a given current density the amount of pumping currentrequired is proportionately lower. One implementation of this solutionis to constrain the pumping current to flow in a relatively narrowchannel (e.g., 12 μm wide) from a major surface of the semiconductorbody through the active region.

One of the earliest structures for constraining current to flow in sucha channel was the stripe geometry contact first proposed forsemiconductor lasers by R. A. Furnanage and D. K. Wilson (U.S. Pat. No.3,363,195 issued on Jan. 9, 1968). The stripe geometry reduces thethreshold current for lasing (compared to lasers with broad areacontacts) and limits the spatial width of the output beam. Since thatearly proposal, numerous laser configurations have been devised toimplement the stripe geometry concept: (1) the oxide stripe laser; (2)the proton bombarded laser; (3)the mesa stripe laser; (4) thereverse-biased p-n junction isolation laser; (5) rib-waveguide lasers;and (6) buried heterostructures of various types.

The most commonly used configuration for the past eleven years, however,has been the proton bombarded, GaAs-AlGaAs double heterostructure (DH)laser described, for example, by H. C. Casey, Jr. and M. B. Panish inHeterostructure Lasers, Part B, pp. 207-210, Academic Press, Inc., N.Y.,N.Y. (1978). Despite its various shortcomings, lasers of this type haveregularly exhibited projected lifetimes in excess of 100,000 hours and anumber have exceeded 1,000,000 hours (based on accelerated aging tests).Long lifetimes have also been projected in DH LEDs employing differentcontact geometries (e.g., dot-shapes or annular rings) but similarproton bombardment to delineate the current channel.

Several of the shortcomings of proton bombarded DH lasers are discussedby R. W. Dixon et al in The Bell System Technical Journal, Vol. 59, No.6, pp. 975-985 (1980). They explored experimentally the opticalnonlinearity (presence of "kinks" in the light-current (L-I)characteristics) and the threshold current distribution of AlGaAs,proton-bombardment-delineated, stripe geometry DH lasers as a functionof stripe width (5, 8, and 12 μm) in cases in which the protons did anddid not penetrate the active layer. They demonstrated that shallowproton bombardment with adequately narrow stripes (e.g., 5 μm) canresult in satisfactory optical linearity (kinks are driven tonon-obtrusive, high current levels) without the threshold penalty thathas been associated with narrow-stripe lasers in which the protonspenetrate the active layer. On the other hand, lasers with such narrowstripes have exhibited a statistically meaningful, although notdemonstrably fundamental, decrease in lifetime. In addition, failure ofthe protons to penetrate the active layer increases device capacitanceand hence reduces speed of response and, moreover, increases lateralcurrent spreading and hence increases spontaneous emission. In digitalsystems, the latter implies a higher modulation current to achieve apredetermined extinction ratio or a lower extinction ratio for apredetermined modulation current.

SUMMARY OF THE INVENTION

We have achieved satisfactorily high optical linearity, low capacitance,and low spontaneous emission levels in stripe geometry, protonbombardment-delineated, GaAs-AlGaAs DH lasers by means of a currentconstraining structure in which the current channel is narrower at thetop near the p-side contact and wider at the bottom near the activelayer. The structure is applicable to other materials systems, to LEDsas well a lasers, and to a variety of configurations other than the DH.

Accordingly, in an illustrative embodiment of a light emittingsemiconductor device of our invention, a semiconductor body comprises anactive region within the body, and constraining means through whichcurrent flows from a surface of the body to the active region, therebycausing radiative recombination of holes and electrons in the activeregion. The constraining means is located within the semiconductor bodyand forms a current flow channel which is narrower at its top near thesurface and wider at its bottom near the active region. In oneembodiment the constraining means forms, in cross-section, atrapezoidal-shaped channel. In another embodiment the constraining meansforms a coupled pair of axial channels of different widths, the narrowerchannel being near the surface and the wider channel being near theactive region.

Another aspect of our invention concerns a particle bombardment methodof making such a device with a trapezoidal channel. The process involvesfirst epitaxially growing a dispensable semiconductor layer on the majorsurface of the body and then exposing the layer to a preferentialetchant which opens inverted trapezoidal stripes in the layer. Theremaining portions of the layer form a trapezoidal attenuation mask (incross-section). When the masked surface is subjected to particle (e.g.,proton, oxygen) bombardment, high resistivity zones are created in theportions of the body between the masks and under the oblique sides ofthe trapezoids. These zones bound the current channel and give it thedesired trapezoidal shape: narrow at the top near the surface and widerat the bottom near the active region. Before metallization of the bodyto form electrical contacts, the mask is removed. To this end the maskis preferably made of a material which is different from the portion ofthe body adjacent the surface so that a stop-etch procedure can beemployed in its removal.

BRIEF DESCRIPTION OF THE DRAWINGS

Our invention, together with its various features and advantages, can bereadily understood from the following more detailed description taken inconjunction with the accompanying drawings. In the interests of clarity,the figures have not been drawn to scale.

FIG. 1 is an isometric view of a light emitting semiconductor devicehaving trapezoidal-shaped current channel in accordance with oneembodiment of our invention;

FIG. 2 is an end view of a semiconductor light emitting device having atrapezoidal-shaped current channel in accordance with another embodimentof our invention;

FIG. 3 is an end view of a semiconductor Hi-Lo light emitting devicehaving a pair of stacked channels in accordance with yet anotherembodiment of our invention;

FIG. 4 is an end view of a mask structure for fabricating a lightemitting device having a trapezoidal current channel in accordance withanother aspect of our invention; and

FIGS. 5 and 6 depict end views of alternative masks for fabricatingdevices according to our invention by means of proton bombardment.

DETAILED DESCRIPTION General Considerations

With reference now to FIG. 1, there is shown a semiconductor lightemitting device (laser or LED) comprising a semiconductor body 11 whichincludes an intermediate region 14. Region 14, which may have one ormore layers, includes an active region which emits radiation 22 whenpumping current is applied thereto. Electrode means, illustrativelycontacts 16 and 18 on body 11, is provided along with a voltage source20 to supply the pumping current. In addition, body 11 includesconstraining means 32 which causes the pumping current to flow in arelatively narrow channel 36 from the top contact 16 through the activeregion after which the current may spread out to bottom contact 18.

Before discussing our invention in detail, it will be helpful to discussfirst the general attributes of a preferred configuration of asemiconductor light emitting device known as a double heterostructure(DH). As shown in FIGS. 1, 2, and 3, a DH comprises first and secondrelatively wide bandgap, opposite conductivity type, semiconductorcladding layers 10 and 12, respectively, and, essentially latticedmatched thereto, intermediate region 14 which is between and contiguouswith the cladding layers. The intermediate region 14 includes a narrowerbandgap active layer, here shown to be coextensive with the region 14,capable of emitting radiation when the cladding layers are forwardbiased. From the standpoint of quantum efficiency, it is well known thatthe active layer is preferably a direct bandgap semiconductor. Layers10, 12, and 14 can be made of materials selected from a number ofsystems; for example, GaAs-AlGaAs or GaAsSb-AlGaAs for operation atshort wavelengths in the 0.7-0.9 μm range approximately, and InP-InGaAsPor InP-AlGaInAs for operation at wavelengths longer than about 1 μm(e.g., 1.1-1.6 μm).

Voltage source 20 forward biases the cladding layers and thereby injectscarriers into the active layer. These carriers recombine to generatespontaneous radiation in the case of an LED and predominantly stimulatedradiation in the case of a laser. In either case, however, the radiationhas a wavelength corresponding to the bandgap of the active layermaterial. Moreover, in the case of a laser or edge-emitting LED, asshown in FIG. 1, the radiation 22 is emitted in the form of a beam alongaxis 23. In the laser the beam is collimated, and axis 23 extendsperpendicular to a pair of resonator mirrors 24 and 26 formedillustratively by cleaved crystal facets or etched surfaces. Thesemirrors constitute optical feedback means for the stimulated radiation.In other applications, for example integrated optics, diffractiongratings may be employed as a substitute for one or both of the mirrors.

Although the electrode means depicted in the laser or edge-emitting LEDof FIG. 1 includes broad area contacts 16 and 18, it is well known inthe art that these contacts can be patterned to form various geometricalshapes. Thus, in the case of transversely-emitting LED, in which thelight output is taken perpendicular to the layers, contact 16 istypically a broad area contact, but contact 18 might be an annular ring(not shown) which accommodates an etched hole (not shown) in one side ofbody 11. Where the bottom portions (e.g., substrate) of body 11 isabsorbing, this etched hole is used to couple radiation to an opticalfiber (not shown) positioned in the hole.

The conductivity type of the active layer is not critical. It may ben-type, p-type, intrinsic or compensated since in typical modes ofoperation under forward bias the number of injected carriers may exceedthe doping level of the active layer. In addition, the intermediateregion 14 may include a multiple of layers which constitute an activeregion, e.g. contiguous p-type and n-type layers of the same bandgapforming a p-n homojunction or of different bandgaps forming a p-nheterojunction. Furthermore, the heterostructure may take onconfigurations other than the simple double heterostructure including,by way of example but without limitation, separate confinementheterostructures as described by I. Hayashi in U.S. Pat. No. 3,691,476,strip buried heterostructures of the type described by R. A. Logan andW. T. Tsang in U.S. Pat. No. 4,190,813, and isotype heterostructures ofthe type described by R. L. Hartman et al in U.S. Pat. No. 4,313,125.

For CW laser operation at room temperature, the thickness of the activelayer is preferably between approximately λ/2 and 1.0 μm, where λ is thewavelength of the radiation as measured in the semiconductor. Foroperation at low thresholds, the thickness is typically 0.12 to 0.20 μm.However, for LED operation a thicker active layer, typically 2 to 3 μm,is suitable. In either case, for room temperature operation the laser orLED is typically bonded to a suitable heat sink, not shown.

In practice, the layers of a double heterostructure are typically grownby an epitaxial process such as liquid phase epitaxy (LPE), molecularbeam epitaxy (MBE), or metallo-organic chemical vapor deposition(MOCVD). Epitaxial growth takes place on a single crystal substrate 28which may include a buffer layer (not shown) between the substrate 28and the first cladding layer 10. Also, as shown in FIGS. 1 and 3 acontact-facilitating layer 30 is optionally included between the secondcladding layer 12 and the top contact 16. The opposite contact 18 isformed on the bottom of substrate 28.

As mentioned previously, in order to constrain the pumping currentgenerated by source 20 to flow in a relatively narrow channel 36 throughthe active region, constraining means 32 is provided within body 11;i.e., high resisitivity zones 32 are formed in the semiconductor layers,illustratively in layers 10, 12, 14, and 30, by means well known in theart. Techniques for forming zones 32 include, for example, protonbombardment, oxygen bombardment, or suitable etching and regrowth ofhigh resistivity material. Illustratively, the zones 32 have aresistivity on the order of 10⁵ -10⁶ Ω-cm, whereas channel 36 has aresistivity of only 0.1 Ω-cm so that typical ratios of resistivity arein the range of 10⁶ :1 to 10⁷ :1.

Trapezoidal Channel Structures

In accordance with an illustrative embodiment of our invention shown inFIG. 1, current constraining means 32 forms a relatively highconductivity current flow channel 36 which is narrower (width S₁) at itstop near major surface 44 and wider (width S₂) at its bottom near theactive region (i.e., layer 14). Constraining means 32 compriseslaterally separate high resistivity regions 32.1 and 32.2 which boundthe channel 36 along its oblique sides 36.1. Although these sides aredepicted as straight lines, in practice a linear relationship is notnecessary and indeed may not result from actual processing techniques.

We have found that the above shape of the current channel has importanteffects on device performance. The narrower channel width at the topincreases the current density and thereby the power at which kinksoccur. The depth of the high resistivity regions, which preferablyextend through the active region 14, affects device capacitance and theamount of spontaneous emission generated in lasers. These matters willbe discussed in greater detail later.

Alternatively, as shown in FIG. 2, the channel 36 formed by highresistivity regions 32.1 and 32.2 need not reach major surface 44.However, in order that the device resistance be not too high, a dopantmay be diffused or otherwise introduced into the surface 44 so as tocreate a highly conducting diffusion front 45 which penetrates channel36. In this case, the width S₁ at the top of the channel 36 is definedby the intersection of the front 45 and the oblique sides 36.1.

The realization of constraining means 32 in accordance with ourinvention need not be limited to configurations in which the channel hasa trapezoidal shape. In the Hi-Lo structures discussed in the nextsection, constraining means 32 may form a coupled pair of stackedchannels.

Moreover, although the trapezoidal channel 36 depicted in FIG. 1constitutes essentially a parallelipiped extending parallel to axis 23,in the case of transversely-emitting LED channel 36 might have the shapeof a truncated cone having its axis perpendicular to the layers.

Hi-Lo Structures

In accordance with this embodiment of our invention shown in FIG. 3,current constraining means 32 has a bi-level or stepped configurationforming a pair of coupled channels 36a and 36b. In particular, means 32includes first means 32.1a-32.2a defining a relatively narrow upperchannel 36a and second means 32.1b-32.2b defining a relatively widerlower channel 36b. Illustratively, the constraining means 32 compriseshigh resistivity regions 32.1-32.2 which bound relatively highconductivity channels 36a and 36b. The regions 32 include upper zones32.1a and 32.2a and lower zones 32.1b and 32.2b. The upper zones areseparated by a relatively narrow distance S₁ and extend from the uppermajor surface 44 of body 11 to a depth d₁ short of the active region,thereby defining the narrow upper channel 36a. In contrast, the lowerzones are separated by a relatively wider distance S₂ >S₁ and extendfrom the depth d₁ into or through the active region (e.g., to a depthd₂), thereby defining the wider lower channel 36b.

As before, channels 36a and 36b may have the approximate shape ofparallelipipeds extending perpendicular to the plane of the paper, as ina laser or edge-emitting LED; or in a transversely-emitting LED may formcylinders extending transverse to the layers.

When the high-resistivity regions 32 were fabricated by protonbombardment in GaAs-AlGaAs lasers, this Hi-Lo structure exhibitedseveral advantages. First, the narrow upper channel 36a increased thecurrent density in the active region and thereby caused kinks to beshifted to satisfactorily high current levels out of the range oftypical laser operation compared to wide (e.g., 12 μm) stripe geometryDH lasers. Second, this feature also resulted in more uniformlydistributed and lower lasing threshold lasers, providing higher yields.Third, because the wider lower channel 36b reduced lateral currentdiffusion and spreading, less spontaneous radiation was emitted outsidethe resonator of the laser, thereby allowing for lower minimummodulation currents for predetermined extinction ratios in digitalapplications. Fourth, the latter feature resulted in reduced devicecapacitance for both lasers and LEDs, thereby permitting high speed ofoperation (i.e., higher pulse repetition rates in digital applications).

To reduce device capacitance the proton bombardment should penetrate thep-n junction which, in a conventional DH, is located at one of theinterfaces between active layer 14 and cladding layers 10 and 12.However, to reduce spontaneous emission, the protons preferablypenetrate through the active region where recombination occurs.

Fabrication of Trapezoidal Channels

As shown in FIG. 4, one way to fabricate a trapezoidal channel of thetype shown in FIG. 1 is to epitaxially grow a removable semiconductorlayer on major surface 44 and by well-known photolithography andpreferential etching techniques to pattern the layer to form invertedtrapezoidal openings 54 which expose portions of surface 44. Between theopenings, the remaining segments 52 of the removable layer formtrapezoidal attenuation masks. For a Group III-V compound semiconductorlayer, the oblique side walls 56 of the remaining segments correspond to(111A) crystallographic planes which make an angle of about 55 degreeswith a (100)-oriented surface 44.

Alternatively, the openings in the removable layer may be etched asinverted trapezoids so that the remaining segments 52 are trapezoids. Ineither case, therefore, the trapezoids and inverted trapezoids arecomplementary.

Bombardment of the masked surface 44 with particles 50 (e.g., protons,oxygen) results in deepest particle penetration between the segments, nopenetration under the central (thickest) parts of the segments, andgradually decreasing penetration under the oblique sides of thesegments. Of course, a thinner mask segment would allow some particlepenetration under the central parts of the segments, a technique whichwould be useful in realizing the channel configuration of FIG. 2.

After bombardment is completed and before metallization to formelectrical contacts, the attenuation masks are removed. To this end, itis preferable that the material of mask 52 be different from thatportion of body 11 adjacent surface 44 so that stop-etch procedures canbe advantageously employed. For example, surface 44 is typically GaAs inwhich case mask 52 could be AlGaAs and a well-known HF etchant or iodineetchant (e.g., 113 g KI, 65 g I₂, 100 cc H₂ O) could be used as astop-etch to remove mask 52. Plasma stop-etching may also be utilized asa substitute for wet-chemical procedures. Finally, it should be notedthat a buffered peroxide solution is also a preferential etchant and canbe used to etch the openings which form mask segments 52.

Formation of the removable layer also gives a fringe benefit related tothe cleanliness of the epitaxial growth process. When liquid phaseepitaxy is used to fabricate the semiconductor layers of these devices,the last grown layer typically gets contaminated from various sources,especially globules of the molten metal (e.g., Ga) used as the sourcesolutions. Consequently, this last layer, which is usually the cap orcontact-facilitating layer 30 (FIGS. 1-3), has to be cleaned by etching,a step which requires careful control since layer 30 is typically verythin (e.g., 0.5 μm). In the process described, here, however, thelast-grown layer is the attenuation mask which can be much thicker(e.g., 3.0 μm) and can be readily removed by stop-etch techniques asmentioned above.

Fabrication of Hi-Lo Structures

A number of fabrication techniques can be employed to fabricate ourHi-Lo structure. As mentioned previously, the high resistivity regions32 can be formed by proton bombardment, oxygen bombardment, or etchingand regrowth of high resistivity material. For purposes of explanation,however, assume that these regions are formed by proton bombardment.

One straightforward technique would entail two proton bombardment stepsand two masks. In the first step a proton attenuation mask S₁ wide andprotons of energy E₁ (e.g., 150 keV) would be used to delineate narrowupper channel 36a. In the second step a proton attenuation mask S₂ wideand protons of energy E₂ >E₁ (e.g., E₂ =300 keV) would be used todelineate wider lower channel 36b.

Delineation of the channels 36a and 36b in a single proton bombardmentstep is also possible. To do so a compound attenuation mask havinghigher proton attenuation in the center and lower attenuation on thesides can be used. Two versions of this type of mask are depicted inFIGS. 5 and 6. In each case a thick metal pad 40 of width S₁ is formedon top of a plateau 42 which in turn is formed on the major surface 44nearest active region 14. Pad 40 essentially totally attenuates theprotons 50 so that no proton damage occurs in the narrow channel 36a,and plateau 42 only partially attenuates the protons 50 so that damagedzones 32.1a and 32.2a extend to a depth d₁ short of the active region.Outside the plateau 42, the mask provides virtually no attenuationeither in FIG. 5 (because the mask does not extend that far) or in FIG.6 (because the mask is very thin there). Thus, outside the plateau 42proton damaged zones 32.1b and 32.2b extend to a depth d₂ and penetratethe active region 14. Preferably, as shown, these damaged zones 32.1band 32.2b extend through the active region 14. Illustratively, in FIGS.5 and 6 pad 40 comprises plated Au. Plateau 42 in FIG. 5 compriseslayers of Au (42.1), Pd or Pt (42.2), and Ti(42.3) and in FIG. 6comprises a mesa of SiO₂ (42.4) overlayed with Ti-Pt layers (42.5).

The following examples describe in more detail how masks of this typewere used to fabricate light emitting devices. Unless otherwise stated,numerical parameters and various materials are given for purposes ofillustration only and are not intended to limit the scope of theinvention. In each of the two examples, the semiconductor body 11comprised a (100)-oriented, n-GaAs substrate 28 on which were grown bystandard LPE the following epitaxial layers: an n-GaAs buffer layer (notshown); a n-Al₀.36 Ga₀.64 As cladding layer 10 about 1.5 μm thick; ap-Al₀.08 Ga₀.92 As active layer 14 about 0.15 μm thick; a p-Al₀.36Ga₀.64 As cladding layer 12 about 1.5 μm thick; and a highly dopedp-GaAs cap layer 30 about 0.5 μm thick. The completed wafer (body 11plus epitaxial layers) was processed as follows to fabricate lightemitting devices, particularly lasers.

EXAMPLE I

To fabricate lasers using the compound attenuation mask 40-42 of FIG. 5,a lift-off photoresist mask was deposited on surface 44, and standardphotolithographic techniques were used to open an elongated stripewindow 12 μm or 18 μm wide perpendicular to the {110} cleavage planes.Ti, Pd, and Au layers 42.3, 42.2, and 42.1 were sequentially depositedusing a vacuum E-gun system. The deposition rate was controlled by acommercially available monitoring system so that the Ti, Pd, and Aulayers had thicknesses of 1000 Angstroms, 1500 Angstroms, and 5000Angstroms, respectively. The total thickness of 0.75 μm for plateau 42was selected to provide 50 percent reduction in the penetration depth of300 keV protons 50. The stripe geometry plateau 42 was then formed bywell-known etching procedures to lift-off the photoresist mask.

Next, the pad 40 was formed also in the shape of a 5 μm wide stripe byelectroplating Au to a thickness of about 1-2 μm using standardphotolithographic procedures. The Au pad 40 provided essentially acomplete barrier to the high energy (300 keV; dosage 3×10¹⁵ cm⁻²)protons 50, thus forming narrow upper channel 36a of width S₁ =5 μm andwider lower channel 36b of width S₂ =12 μm or 18 μm. Between S₁ and S₂the plateau 42 provided only partial attenuation so that protonspenetrated to a depth d₁ =1.5 μm. Outside S₂ no attenuation mask waspresent, and protons penetrated to a depth d₂ =2.8 μm and hence extendedthrough the active layer 14.

EXAMPLE II

To simplify the fabrication procedure of Example I, the Ti-Pd-Au plateau42 was replaced as shown in FIG. 6 with dielectric stripe 42.4 (e.g.,SiO₂ or Si₃ N₄) overlayed with Ti-Pt layer 42.5. This compound mask wasmade by depositing about 1.0-1.2 μm of SiO₂ on surface 44 using standardvapor phase techniques. This thickness was again chosen to provide 50percent attenuation to the 300 keV protons 50. Next, the SiO₂ layer wasphotolithographically delineated and etched in standard buffered HFetchant to form stripes 12 μm or 18 μm wide perpendicular to the {110}cleavage planes. After removing the photolithography mask, the SiO₂stripe 42.4 and the surface 44 were covered with 1000 Angstroms of Tiand then 1500 Angstroms of Pt by standard evaporation procedures.Finally, pad 40 was formed in the shape of a 5 μm stripe 1-2 μm thickusing standard photolithography and electroplating techniques. Asbefore, the masked wafers were subjected to 300 keV protons in a dosageof 3×10¹⁵ cm⁻² to form simultaneously the narrow upper channel 36a andthe wider lower channel 36b. In this case layer 42.5 reduced the protonenergy so that d₂ decreased to about 2.3 μm.

In both Examples I and II, after proton bombardment was completed, thecompound masks 40-42 were removed from surface 44 by means of an HFetchant. This step also prepared the surface 44 for subsequentmetallization to form standard p-metal contacts.

EXPERIMENTAL RESULTS--HI-LO STRUCTURES

In order to provide a standard for comparison, one half of each wafer inExamples I and II was processed into control lasers having 5 μm widestripes with shallow proton bombardment (150 keV). Each remaining halfwafer was processed as above into Hi-Lo lasers using compound masks40-42 of three types: Type (1)--5 μm wide Au pad 40 on 18 μm wide SiO₂/Ti-Pt plateau 42 (Example II); Type (2)--5 μm wide Au pad 40 on 12 μmwide SiO₂ /Ti-Pt plateau 42 (Example II); and Type (3)--5 μm wide Au pad40 on 18 μm wide Ti-Pd-Au plateau 40 (Example I).

Comparisons set forth in the table below were based on a number ofparameters: spontaneous emission power S_(L) at 50 mA of drive current;slope ΔS_(L) of spontaneous emission portion of the L-I curve;capacitance C measured at 1 MH_(z) (average C is listed below); andminimum modulation current MMI which is defined as the difference incurrent between upper and lower light power levels P₂ and P₁,respectively, which yield a light intensity extinction ratio E_(R)between ON and OFF states when the laser is pulsed (the median MMI islisted below for E_(R) =15:1, P₂ =2.5 mW, and P₁ =0.167 mW).

    ______________________________________                                        LASER TYPE                                                                             Con-    Type    Con-  Type  Con-  Type                               Parameter                                                                              trol    (1)     trol  (2)   trol  (3)                                ______________________________________                                        S.sub.L (mW)                                                                           0.200   0.151   0.142 0.042 0.071 0.041                              ΔS.sub.L (mW)                                                                    0.29    0.28    0.20  0.07  0.12  0.06                               MMI (mA) 59      45      76.5  24    70.5  26                                 C (pf)   83      21      115   35    54    12                                 ______________________________________                                    

In addition to the data shown in the table, we found that 90 percent ofthe Type (2) lasers had MMIs within a specified 30 mA MMI with astatistical variance 2σ≃3 mA. Similarly, 75 percent of the Type (3)lasers had MMI within 30 mA whereas none of the corresponding controllasers did. These results imply improved device yield.

Note that the Type (2) lasers, which have S₂ =12 μm wide stripes,exhibit the largest decrease in S_(L) and the highest yield for anMMI≦30 mA, but these advantages alone do not necessarily dictate the useof this stripe width. Consideration should be given to the impact on thelight power output level P_(k) at which kinks occur. In general, wefound that kink formation occurred at higher P_(k) in the control lasersthan in Hi-Lo lasers, but the latter were still well withinspecifications (i.e., P_(k) ≧3 mW). Type (1) lasers showed little changein P_(k). However, the Type (2) lasers, which utilized the narrowestattenuation masks (S₂ =12 μm), showed a marked reduction of about 50percent in P_(k) compared to corresponding control lasers. In contrast,Type (3) lasers, which had S₂ =18 μm, had a smaller reduction of about35 percent in P_(k). This data suggests that it may be advantageous forthe width S₂ to be between 12 μm and 18 μm.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, as described in theconcurrently filed application of R. L. Hartman et al, supra, the firstmeans defining the narrow upper channel 36a can be realized by means ofa groove etched into the upper surface 44. Consequently, a groove incombination with a wider lower chanel 36b is expected to have featuresand advantages comparable to those described above.

What is claimed is:
 1. In a semiconductor light emitting device, asemiconductor body comprisingan active region in which optical radiationis generated when current flows therethrough, and means within said bodyfor constraining said current to flow from a surface of said body in achannel through said active region, said channel being narrow near saidsurface and wider near said active region.
 2. The body of claim 1whereinsaid constraining means comprisesfirst means for causing saidcurrent to flow in relatively narrow upper channel which extends fromsaid surface to a depth short of said active region, and second meansfor causing said current flow in relatively wider lower channel whichextends from said depth to said active region.
 3. The body of claim 2wherein said second means comprises a pair of high resistivity secondzones bounding said lower channel.
 4. The body of claim 3 wherein saidsecond zones extend through said active region.
 5. The body of claim 3wherein said first means comprises a pair of high resistivity firstzones bounding said upper channel.
 6. The body of claims 3, 4, or 5wherein said high resistivity zones comprises proton bombarded zones. 7.The body of claims 3, 4, or 5 wherein said upper channel is about 5 μmwide and said lower channel is between about 12 and 18 μm wide.
 8. Thebody of claims 3, 4, or 5 comprisinga first cladding layer, a secondcladding layer nearer said surface than said first layer, said activeregion comprising an active layer between said cladding layers, andwherein said upper channel extends from said surface to said depthlocated in said second cladding layer, and said lower channel extendsfrom said depth through said active layer.
 9. The body of claim 8 foruse in a laser having a resonator axis along which radiation propagatesand wherein said constraining means defines said channels as elongatedparallelipipeds extending essentially parallel to said axis.
 10. Thebody of claim 8 for use in a light emitting diode and wherein saidconstraining means defines said channels as cylinders extendingtransverse to said layers.
 11. A double heterostructure semiconductorlaser comprisinga semiconductor body having a major surface andincluding first and second cladding layers and an active layertherebetween, electrode means for applying pumping current to flow fromsaid surface through said active layer, thereby resulting in theemission of stimulated radiation from said active layer, opticalfeedback means for resonating said radiation along an axis parallel tosaid layers, and means within said body for constraining said pumpingcurrent to flow in a channel from said major surface through said activelayer, said constraining means comprisingfirst means within said bodyfor causing said current to flow in a relatively narrow upper channelwhich extends from said major surface to a depth short of said activelayer, and second means within said body for causing said current toflow in a relatively wider lower channel which extends from said depththrough said active layer.
 12. The laser of claim 11 whereinsaid firstmeans comprises a pair of laterally separate, high resistivity, protonbombarded first zones bounding said upper channel, and said second meanscomprises a pair of laterally separate, high resistivity, protonbombarded second zones bounding said lower channel.
 13. The laser ofclaim 12 wherein said first channel is about 5 μm wide and said secondchannel is between about 12 and 18 μm wide.